Elemental phosphorus is produced by smelting a mixture of phosphate ore, reducing carbon, and silica in a submerged-arc electric furnace. The ore reacts with the reducing carbon at high temperature and in this reaction the combined phosphorus is reduced to the element. Some of the metal oxides present in the furnace feed materials are reduced to metallic elements. Iron oxide is a constituent in the furnace feed materials and this oxide is reduced to elemental iron which combines with elemental phosphorus to form a mixture of iron phosphides called ferrophosphorus. Other metals alloy with the ferrophosphorus. The ferrophosphorus is tapped from the furnace as a molten material.
Phosphorus furnace feed materials contain metal oxides which are not reduced by carbon to form the metal elements. The metal oxides combine to form a slag which is tapped from the furnace as a molten material. Both slag and ferrophosphorus are byproducts.
Hydrogen compounds react with reducing carbon in the furnace to form hydrogen gas. The hydrogen compounds are hydrocarbons which may be constituents of the reducing carbon. Furnace feed materials may contain moisture or combined water and this compound is reduced to form hydrogen gas. The hydrogen mixes with other gases volatilized from the furnace.
Carbon in the reducing carbon oxidizes to form carbon monoxide when the various compounds in the furnace are reduced. The carbon monoxide mixes with the other gases volatilized from the furnace. Most of the carbon monoxide is derived from the reduction of combined phosphorus in phosphate ore and only a small proportion is formed by the reduction of metal oxides and hydrogen compounds. Most of the elemental phosphorus formed by the reduction of phosphate ore is discharged from the furnace as a gas. Therefore, gases from the furnace are composed mainly of carbon monoxide and phosphorus. But the furnace gas contains other materials. Sources of other materials in the furnace gas mixture are given below.
1. Reducing carbons fed to the phosphorus furnace contain volatile constituents, and some of these constituents are volatilized without being reduced. Both saturated and unsaturated hydrocarbons are volatilized. Some reducing carbons, such as coals, contain combined water which may be reduced in the furnace to form hydrogen.
2. Phosphate ore contains fluorine, and part of the fluorine is volatilized when the ore is heated in the phosphorus furnace. The fluorine probably is discharged from the furnace as silicon tetrafluoride (SiF.sub.4).
3. Air enters the furnace through feed chutes when materials are fed into the furnace. Also, air enters the furnace through openings in the furnace roof. Oxygen in the air oxidizes the elemental phosphorus to P.sub.2 O.sub.5 and the P.sub.2 O.sub.5 mixes with the furnace gas stream. Nitrogen also mixes with the furnace gas.
4. Small particles of furnace feed materials become suspended in the gas.
5. Metals and metal compounds are volatilized by the electric arc in the furnace. These materials condense as small particles which become suspended in the furnace gas stream.
Furnace gases are treated to condense the elemental phosphorus from the gas mixture, thereby separating the phosphorus from the other gases. FIG. 1 in patent application Ser. No. 503,099 illustrates the process for treating phosphorus furnace gas stream. The gases are treated in an electrostatic precipitator to remove particulates. A small amount of elemental phosphorus is collected with the particulates. The gases are then contacted with water to condense phosphorus by adiabatic cooling. The gases are further cooled by water in a tubular cooler to condense additional phosphorus. Then the gases are exhausted by a compressor.
The composition of the condenser exhaust gas depends largely on the type and composition of the reducing carbon used in the furnace. Metallurgical coke is frequently used as a reducing carbon. The analyses shown in Table 1 were obtained when metallurgical coke was being used.
TABLE 1 ______________________________________ Composition of Condenser Exhaust Gas.sup.a Percent by volume Constituent on a dry basis ______________________________________ CO.sub.2 0.7 O.sub.2 0.1 CO 87.3 H.sub.2 7.9 N.sub.2 2.3 Unsaturated 0.6 hydrocarbons.sup.b CH.sub.4 1.1 ______________________________________ .sup.a Metallurgical coke was the reducing carbon. .sup.b Assumed to be ethylene.
The gross heating value of the gas was calculated to be 343 Btu per cubic foot at STP. About 80,500 cubic feet (STP) of gas was obtained per ton of phosphorus produced. The potential energy in the gas was about 27.6 million Btu per ton of phosphorus produced. Energy in the metallurgical coke was about 38.2 million Btu per ton of phosphorus, and the condenser exhaust gas has about 72 percent as much energy as the reducing carbon.
The furnace gas contains about 24.9 pounds of elemental phosphorus per 1,000 cubic feet of noncondensable gas at STP, and this is equivalent to about 6 percent phosphorus by volume. Dust is collected in the precipitator at a rate of about 0.07 ton per ton of elemental phosphorus produced. The gas mixture is essentially dry, but it is cooled from about 600.degree. F. to about 145.degree. F. by saturating with water vapor in a spray chamber. Phosphorus content of the gas is reduced to about 0.3 pound per 1,000 cubic feet of noncondensable gas at STP by cooling in the spray chamber. About 98.8 percent of the phosphorus in the furnace gas is recovered by adiabatic cooling.
The gases are further cooled in a condenser wherein gases flow through the tubes and cooling water is outside. Water is sprayed into the tubes to irrigate the inside surfaces and prevent fouling. The gases are cooled to about 128.degree. F. in the tubular cooler and the phosphorus content of the gases is reduced to about 0.2 pound per 1,000 cubic feet of noncondensable gas at STP, resulting in an overall phosphorus recovery of about 99.2 percent. Water is condensed in the tubular cooler at a rate of 484 pounds per ton of phosphorus produced. After condensation of the phosphorus the gases are exhausted by a Nash Hytor pump.
Both the adiabatic condenser and the tubular cooler drain to a sump as shown in FIG. 1, application serial No. 503,099. Condensate from the condensers is a mixture of water, liquid phosphorus, and an impure phosphorus product called phosphorus sludge. The sump is designed to separate the three materials as shown in FIG. 91 in Chemical Engineering Report No. 3. Two partitions are in the sump, thereby dividing it into three compartments. Liquid phosphorus has a specific gravity of 1.73 at 140.degree. F. and it separates from other materials by collecting in the compartment under the condenser drain. The liquid phosphorus is periodically pumped to storage tanks by submerged pumps. Water and phosphorus sludge overflow a partition and flow into the middle compartment and phosphorus sludge collects in the bottom of this compartment because it has a specific gravity of about 1.23. The phosphorus sludge is periodically removed from the compartment by submerged pumps and is stored in tanks. Water overflows the second partition and enters the third compartment called the spray liquor compartment. From this compartment the water is recirculated to the Nash Hytor pump, the tubular cooler, and the adiabatic condenser by pumps.
The electrostatic precipitators at phosphorus furnaces are provided to collect solid particles that become entrained in the gas stream. From 60 to 90 percent of the entrained solids are collected. Uncollected solids are removed from the gas stream by contacting the gases with water sprays in the adiabatic condenser and the tubular cooler.
Phosphorus sludge becomes viscous after storage. Also, the material may be sticky depending on the type of reducing carbon used. Microscopic examination of the sludge shows that it consists of globular particles of yellow phosphorus as large as 1 to 2 millimeters in diameter and the particles may be only a few microns in diameter. Some of the smaller globules agglomerate into clusters. Electrical charges and physical barriers of foreign solids are thought to retard or prevent coalescence of the phosphorus particles.
Following is the approximate composition of the major solid constituents in the sludge.
______________________________________ P.sub.2 O.sub.5 32 percent F 23 percent CaO 8 percent SiO.sub.2 7 percent ______________________________________
The proportion of total phosphorus produced as phosphorus sludge varies widely, but data given in Chemical Engineering Report No. 3, Tables X and XII, may be used to determine a typical proportion, as indicated below.
______________________________________ Tons elemental phosphorus per day ______________________________________ 22.4 tons per day of phosphorus = 21.91 containing 97.8 percent phosphorus 5.5 tons per day of phosphorus sludge = 3.79 containing 68.9 percent phosphorus Total 25.70 ______________________________________
The data above indicate 14.7 percent of the phosphorus is collected as phosphorus sludge when separation is made in the condenser sump. The phosphorus content was 68.9 percent and this was rich enough to burn in a phosphoric acid production unit.
When phosphorus sludge is stored, heated, or agitated, some of the particles of yellow phosphorus coalesce. When the material is stored in a tank, liquid phosphorus will collect in the bottom of the tank. The phosphorus can be pumped out and the yield of good phosphorus is thereby increased. When this occurs the percentages of solids and water increase. Upon prolonged storage with occasional melting and agitation, the phosphorus content of the sludge may be reduced to the range of 5 to 15 percent. The material becomes very viscous and the stickiness increases. Material with depleted phosphorus content is called consolidated sludge, and it possesses characteristics which prevent it from being pumped.
In addition to storing, heating, and agitating, the recovery of good phosphorus can be increased by filtering or centrifuging the phosphorus sludge. In both cases residues are obtained and the residues contain too much phosphorus to be discarded. When liquid or solid materials contain elemental phosphorus, serious pollution problems arise when the materials are stored or discharged as wastes. Furthermore, filtering or centrifuging phosphorus sludge is both hazardous and costly.
The phosphorus in phosphorus sludge can be evaporated by heating the material in a still. A carrier gas can be provided to sweep out the phosphorus in the still and the phosphorus can be separated from the carrier gas by condensing. The gas mixture can be cooled by contacting it with water by an arrangement similar to that shown in FIG. 1, application Ser. No. 503,099. A still is shown as FIG. 4 in the present application. Prolonged heating of sludge in the still will evaporate essentially all the phosphorus from the solids, but a large amount of fuel is required.
At the Tennessee Valley Authority National Fertilizer Development Center, phosphorus sludge was burned and P.sub.2 O.sub.5 was formed. The P.sub.2 O.sub.5 was reacted with water to produce phosphoric acid in accordance with a process described in Corrosion, Volume 14, 21-6, August 1958. Operation of the phosphoric acid unit with various compositions of phosphorus sludge disclosed that the minimum phosphorus content required for combustion was about 60 percent. Freshly made sludge normally contained enough phosphorus to burn, as discussed above, but consolidated sludge or residues obtained by filtering or centrifuging were too lean to burn. Dust is discharged from precipitators as a slurry and the slurry contains elemental phosphorus. However, the phosphorus content of the precipitator dust is too low for the material to be burned to produce phosphoric acid. When the sludge contained less than about 60 percent phosphorus the material had to be enriched with phosphorus to increase its calorific value. Treatment of sludge to increase the recovery of good phosphorus was of little benefit.
The phosphoric acid produced by burning phosphorus sludge is black and it contains solid impurities. Only orthophosphoric acid could be produced because of the relatively low calorific value of the sludge. The phosphoric acid was used for the production of fertilizers, and non-orthophosphoric acid was preferred. Phosphorus in sludge was not completely oxidized to P.sub.2 O.sub.5. Some of the phosphorus was oxidized to P.sub.2 O.sub.3 and a mixture of phosphoric and phosphorous acids was formed. The value of phosphoric acid made from sludge was less than that made from good phosphorus. Consequently, the alternative of burning phosphorus sludge to produce impure phosphoric acid is not attractive.
At times during World War II, all the phosphorus produced at the TVA National Fertilizer Development Center was used in munitions. None of the phosphorus was converted into phosphoric acid. During these periods the phosphorus and phosphorus sludge were agitated and elutriated with hot water in equipment called a washer. High-quality phosphorus was produced as required for munitions. Solid impurities and phosphorus droplets overflowed the washer. This watery mixture was discharged into a 14-acre settling pond to clarify the water before it was discharged into a watercourse.
Clarified water contained dissolved elemental phosphorus and colloidal phosphorus particles in suspension. The clarified water was toxic to marine animals, and fish were unable to survive in the receiving stream. Breakouts occurred at the settling pond, resulting in the release of additional hazardous waste to the watercourse. Phosphorus production was discontinued at the TVA National Fertilizer Development Center in 1976. Nevertheless, the settling pond continues to be a potential pollution hazard. Additional breakouts of phosphorus-containing solids may occur and water in the settling pond may be inadvertently discharged to the receiving stream. Heretofore, technology has not been available to dispose of the hazardous wastes.
Wastes generated at phosphorus furnaces are safety hazards and they may cause serious air and water pollution problems. Most of the wastes contain elemental phosphorus which oxidizes spontaneously when the waste is exposed to air. A phosphorus oxide, P.sub.2 O.sub.5, is emitted as small sized particulates which are readily airborne. The P.sub.2 O.sub.5 emitted to the air in this manner causes air pollution. Marine animals are poisoned when watercourses contain small concentrations of elemental phosphorus. Some species are killed when the watercourse contains only 1 to 2 parts of elemental phosphorus per billion parts of water, and it is obvious that release of a small amount of phosphorus-containing waste can cause serious water pollution problems.
Elemental phosphorus and phosphorus-containing wastes are kept underwater to prevent exposure to air and oxidation of the phosphorus. Water exposed to phosphorus in this manner becomes a potential water pollutant. The solubility of phosphorus in water is about three parts of phosphorus per million parts of water, or about 3,000 times the toxic limit. Furthermore, colloidal phosphorus particles are suspended in water which has come in contact with phosphorus. The colloidal particles are not removed by clarification. Solid waste disposal processes should include treatment of water which has come in contact with phosphorus or phosphorus-containing materials.
Chemical processes are available for treating water to remove elemental phosphorus. In most cases the element is oxidized, but air pollution may be a problem if the resulting P.sub.2 O.sub.5 is released to the atmosphere. Chemical treatment of the water usually is economical because large quantities of the water must be treated. Further investigation of the disposal of water waste containing elemental phosphorus led to two inventions for recovery of the water by adding it to suspension fertilizers, as disclosed in U.S. Pat. Nos. 4,383,847 and 4,451,277. When wastewater from phosphorus condensers is recovered, a saving of about $2.00 per ton of phosphorus is obtained because the wastewater contains nutrients.
When hazardous wastes are converted into useable products, some benefits accrue which lessen the overall problem. In the case of phosphorus-containing wastes an obvious product is elemental phosphorus. In the present invention consideration was given to the preparation of products which can be recycled to the furnace as feed materials. The material is smelted and combined phosphorus is recovered as elemental phosphorus. Some of the benefits from treatment of the phosphorus-containing waste in this manner are listed below.
1. Complete removal of elemental phosphorus from the waste will not be necessary.
2. Constituents in the solid wastes have value as furnace feed.
3. Toxic metals in the wastes become encapsulated in the glassy slag, or these metals alloy with ferrophosphorus. In both cases the toxic metals are rendered innocuous and they are ultimately disposed of as byproducts.
4. Phosphorus-containing wastes can be converted into useable products at the rate they are generated. Accumulation of the toxic wastes at the plant site can be eliminated.
An object of the present invention is to recover the wastes by converting them into useable products, and a further objective is to provide recovery processes which comply with all environmental requirements. A further object is to recover the wastes at minimum net cost of production.
The recovery processes are expected to be applied at the following sites.
1. Phosphorus furnace plants producing high-quality phosphoric acid.
2. Phosphorus furnace plants producing high-quality elemental phosphorus.
3. Plant sites where neither phosphoric acid nor elemental phosphorus is produced.
About 80 percent of the elemental phosphorus produced in the U.S. is converted into phosphoric acid. In the process, elemental phosphorus is burned to oxidize it to P.sub.2 O.sub.5. The oxide is then reacted with water to produce phosphoric acid. Under some conditions phosphorus-containing wastes can be burned to oxidize the elemental phosphorus to P.sub.2 O.sub.5 which can be reacted with water to produce phosphoric acid. However, phosphoric acid made from phosphorus-containing wastes is highly contaminated. Furthermore, the wastes are not readily burned because solids in the wastes interfere with combustion.
Phosphorus sludge usually contains enough phosphorus to sustain combustion, but the phosphoric acid made from it is contaminated with fluorine and suspended solids. The solids in phosphorus sludge interfere with combustion as they do in the phosphorus-containing wastes, and phosphorus is frequently incompletely oxidized. The acid produced may be a mixture of phosphoric and phosphorous acids. The value of phosphoric acid made from phosphorus sludge is adversely affected by contamination, and the value may be further decreased by the presence of phosphorous acid or other acids which contain incompletely oxidized phosphorus. The recovery of phosphorus sludge as phosphoric acid has not been practiced widely because of the economic penalty from the production of a low-value product.
Phosphoric acid can be used to agglomerate phosphate ore as disclosed in U.S. Pat. No. 4,372,929. Agglomeration is not adversely affected by the use of highly contaminated phosphoric acid. Furthermore, phosphorus in the acid may be incompletely oxidized, and the use of a mixture of phosphoric and phosphorous acids produced by burning phosphorus sludge is unimportant when the acid is to be used for agglomeration. In the agglomeration process the acid is mixed with phosphate ore and an alkaline substance. The mixture is then tumbled to form agglomerates and the agglomerates are dried to indurate the material. Phosphate ore is normally agglomerated by heating it to temperatures high enough to partially melt it. Also, the ore may be agglomerated and then indurated by heating the agglomerates to high temperature. In both cases the phosphate ore undergoes phase changes by heating to high temperatures. But the phosphate ore undergoes no change in phase when it is agglomerated according to the processes described in U.S. Pat. No. 4,372,929, and this is called low-temperature agglomeration. When the ore undergoes phase changes, the process is called high-temperature agglomeration. The energy consumed is much greater for the high-temperature process than for low-temperature agglomeration.
In U.S. Pat. No. 4,421,521 a process is described for the agglomeration of reducing carbon by the low-temperature process. Phosphoric acid sludge is mixed with the reducing carbon instead of phosphoric acid. The mixture is neutralized with an alkaline substance, the mixture is tumbled to form agglomerates, and the agglomerates are indurated by drying.
Phosphoric acid sludge used in the agglomeration process is a heterogeneous mixture which has settled from wet-process phosphoric acid. Much of the material is solids which have precipitated from merchant-grade wet-process phosphoric acid. A typical analysis of phosphoric acid is given in table 2.
TABLE 2 ______________________________________ Typical Analysis of Phosphoric Acid Sludge Constituent Percent ______________________________________ P.sub.2 O.sub.5 45.6 Fe.sub.2 O.sub.3 1.9 Al.sub.2 O.sub.3 1.5 MgO 0.8 F 2.3 SO.sub.4 7.8 CaO 3.0 H.sub.2 O 18.2 Solids 18-19 Total 99.1-100.1 ______________________________________
From the data in table 2 it is obvious that the phosphoric acid sludge is highly contaminated. Nevertheless, the material could be used satisfactorily to agglomerate reducing carbon particles.
Elemental phosphorus is soluble in benzene, and phosphorus sludge is extracted with benzene as a part of the analysis to determine the elemental phosphorus content of the material. Some organic chemicals may be present which may dissolve in benzene. The benzene extract is normally analyzed for phosphorus to complete the determination. The material insoluble in benzene is called "B.I."; the B.I. in phosphorus sludge contains P.sub.2 O.sub.5, F, CaO, SiO.sub.2, and carbon as major phases. High-quality phosphorus contains only about 0.5 percent B.I. The phosphorus sludge collected in condenser sumps typically contains 7 to 22 percent B.I., and its phosphorus content is typically in the range of 70 to 90 percent. But some of the phosphorus separates out during storage and the B.I. content of the material increases. Phosphorus sludge stored for a prolonged period and occasionally agitated by pumping may contain only about 5 percent phosphorus on a dry basis and the remainder will be B.I. when the sludge contains no significant concentration of organic materials. Phosphorus sludge with such low concentrations of elemental phosphorus is uneconomical to process to recover the phosphorus values, and the material may be considered a waste.
From 15 to 20 percent of the phosphorus produced is recovered as phosphorus sludge in the condenser sump. However, the proportion recovered as phosphorus sludge depends on the performance of the electrostatic precipitator. Precipitators are not provided at some furnaces, and the quantity of sludge collected will be greater than it is at furnaces equipped with precipitators.
Phosphorus sludge normally has a calorific value high enough to burn. When phosphoric acid is made from the material the ratio of solids to P.sub.2 O.sub.5 will be in the range of 0.02 to 0.09, but the ratio of solids to P.sub.2 O.sub.5 in the phosphoric acid sludge is considerably higher; consequently, it was assumed that the phosphoric acid made from phosphorus sludge is satisfactory for use in low-temperature agglomeration of phosphate ore. A process is therefore provided for recycling phosphorus sludge, and said process consists of the following steps.
1. Burning phosphorus sludge to make a mixture of phosphorus oxides.
2. Reacting the mixture of phosphorus oxides with water to make an acidic mixture.
3. Recycling acidic mixture prepared in 2 to low-temperature agglomerating processes wherein phosphate ore is prepared for smelting.
4. Feeding agglomerated phosphate ore prepared in 3 to a submerged-arc electric furnace.
5. Smelting agglomerated phosphate ore to produce phosphorus and phosphorus sludge.
Small sized reducing carbon can be agglomerated by the low-temperature process. When the agglomerated reducing carbon is fed to the furnace the carbon reduces phosphate ore and combined phosphorus in the agglomerates to produce elemental phosphorus.
At the TVA National Fertilizer Development Center, phosphorus sludge was burned and the phosphorus oxides were hydrated to make phosphoric acid. The facility is shown in FIG. 3 in the publication, "Corrosion Problems in the Manufacture of Phosphoric Acid from Elemental Phosphorus," Corrosion 14, 21-6 (August 1958). About 20 percent of the phosphorus in the sludge was burned in the vaporizer. Unburned phosphorus was vaporized and it flowed to a combustion chamber, called a burner in the publication, and combustion was completed. Part of the B.I. in the phosphorus sludge remained in the vaporizer where it accumulated as a slag. Nevertheless, the quality of phosphoric acid made from phosphorus sludge was much lower than that made from high-quality phosphorus. The acid contained carbon and it was black, the fluorine content was greater than acid made from high-quality phosphorus, and a small percentage of the phosphorus in the acid was incompletely oxidized. The phosphoric acid was used to make some fertilizer mixtures, but it was not satisfactory for making chemicals such as feedgrade phosphates. But the process for recycling phosphorus sludge described above does provide for all the phosphorus to be converted into high-quality phosphoric acid. Phosphorus sludge is not stored, and loss of phosphorus in vitiated sludge will be eliminated.
One of the methods for treating phosphorus sludge to recover high-quality phosphorus is to centrifuge the material. A residue is obtained which is a solid waste, but separation of phosphorus from B.I. is not complete and some elemental phosphorus will remain in the residue. The elemental phosphorus can be removed by heating the residue in a still to evaporate the phosphorus. The waste may be disposed of in a landfill provided no toxic metals are present. However, toxic metals may be volatilized in the furnace and condensed as finely divided particles. The particles may be carried over into the condensing system where they will collect in the B.I. Landfill disposal is not satisfactory when centrifuge residue contains toxic metals.
Centrifuging of phosphorus sludge and evaporating the phosphorus in a still are costly treatment methods. The phosphorus-containing wastes frequently are stored in tanks, sumps, or ponds to avoid treatments which do not yield enough phosphorus to pay the treating cost. Fumes may be emitted from the storage facilities, and accidental discharges of the phosphorus-containing wastes to watercourses are continuous environmental hazards.
Phosphorus sludge may be filtered to separate high-quality phosphorus from the B.I. material. Unrecovered phosphorus remains in the filter cake, and the problems of treating and storing the filter cake waste are similar to those in centrifuging.
Costs and hazards associated with centrifuging and filtering phosphorus sludge are eliminated by converting the material into an acidic mixture and recycling the mixture to a low-temperature agglomerator. Phosphorus-containing wastes, such as residue from centrifuging and filter cake from filtering, are not generated and treatment of these wastes is eliminated.